Wrestled w/this myself. Didn't want to purchase PS. Picasa wouldn't do this
so I dumped it and now use paint.net. Open one of your pictures in Paint.net
and rename it so you don't overwrite original. I usually open the picture in
photo gallery in vista or paint in xp and copy it. Then I go back to
paint.net program and choose paste as new layer. Then move new layer around
and play w/transparency settings if you want an overlay of the image. There
may be a better way than this, but I've created a bunch of cool collages
since using this method, as I had to figure it out quickly.
Here's their site:
http://www.getpaint.net/index.html. Maybe check out
tutorials and discussion groups before dnldg to make sure it meets your needs.